Dump type water blasting guns, of the type described by U.S. Pat. No. 5,349,982, are operated by closing the trigger, which in turn causes a valve to close. This results in the sealing off of the relatively large dump passage and forces water to flow through a much smaller nozzle orifice at a much greater pressure than the pressure at which it was previously dumping. The dump valve is biased away from its seating position by a spring. If the trigger is released, the valve will open and the former high pressure flow will, instead, dump to waste, and the pressure in the system will drop to a much lower level. If the spring acting on the valve were to break, or if dirt or some other contaminant were to accumulate in the narrow annulus between the valve stem and the guide bore in which it slides, the valve could jam. This would result in a potentially dangerous operating condition for the operator, since he would have no immediate means for releasing the high pressure, other than by shutting down the water source. Since the source may be remote from the operator, no safe viable alternative may be available. The subject matter of this invention addresses this particular problem.